Pressurization System Flashcards
How does pressurization in an aircraft work?
The airplane’s cabin is sealed, and air is pumped in to maintain internal pressure as close as possible to that at sea level.
In turbine aircraft, a steady supply of engine bleed air is used to pressurize the cabin. Cabin pressure is then controlled by modulating the exhaust of cabin air via outflow valves. Outflow valves are controlled by a pressurization controller operated by the pilot.
What is maximum differential / max diff?
Maximum differential / max diff is the maximum ratio of cabin pressure to outside air pressure that the pressurization system and aircraft can sustain. It is the main measure of a pressurization system’s efficiency.
Max diff varies a lot by aircraft type. This is due to many factors, including pressurization design, engine bleed air capacity, aircraft weight and power.
For many pressurized aircraft, certified maximum operating altitude is determined not by the airplane’s service ceiling but by the ability of the pressurization to meet supplemental oxygen requirements of the FARs.
Maximum operating altitudes, in such cases, are defined by the greatest altitude the aircraft can attain and still maintain legal cabin altitudes, which are 10,000 feet for Parts 135 and 121, and 12,500 feet for Part 91.
What are the three indicators pilots use to monitor pressurization?
- Cabin altitude
- Cabin rate of climb
- Pressure differential
What does the pressure relief valve do?
The pressure relief valve acts as a safety valve and releases any excess pressure to prevent over pressurization.
Aircraft are built to withstand higher pressure on the inside without damage – think of blowing up a balloon without it popping – but there is a limit to how much pressure they’re designed to take. Thus the positive pressure relief valve opens if the pressure inside becomes greater than desired to keep it from reaching the point where damage could occur.
This is most likely to happen if the outflow valve malfunctions and stays closed when it shouldn’t.
What do the auto, standby, and manual modes do on an pressurization system?
Auto - A computer-controlled “auto” mode requires little pilot input. It uses information from an air data computer and cabin sensors to automatically control all aspects of the pressurization cycle.
Standby - Allows semiautomatic control of cabin pressure if the auto mode computer is not operating correctly.
Manual - Bypasses the electronic pressurization controller entirely, sometimes using vacuum or low-pressure air to control the outflow valves directly.
What does the negative pressure relief valve do?
The negative pressure relief valve makes sure the cabin pressure never falls below the outside atmospheric pressure.
Aircraft are not designed so robust as far as pressures outside being greater than pressures inside go – think of the “crushed can” demonstration in science class.
Having greater pressure outside the fuselage than inside is considered “negative pressure” and that’s a bad thing… thus the negative pressure relief valve opens at a far lower differential than the positive pressure relief valve.
Negative pressure relief valves come into play when the pilots forget, upon initiating descent, to set the controller for landing. Say that the airplane is cruising at 25,000 feet, with a cabin altitude of 4,000 feet.
We’ll assume a sea-level destination. When properly set for descent, the controller gradually descends the cabin at a few hundred feet per minute, while the plane itself may be descending at several thousand feet per minute. However, if the controller is not reset for landing, the cabin will stay at 4,000 feet until the airplane reaches the matching 4,000-foot pressure altitude.
From that point on, the negative pressure relief valves will vent the cabin to prevent cabin pressure from becoming lower than outside air pressure.
This situation, known as “catching the cabin,” is not particularly serious, except that the cabin’s descent rate now matches that of the airplane.
In a fast airplane, high descent rates are required to get down in the same distance as a slower plane. Descent rates of 3,000 fpm are common in turbine aircraft. Catching the cabin, therefore, makes it difficult to keep both ATC and the passengers’ ears happy.
What do dump valves do?
Dump valves allow pilots to manually vent cabin pressurization in an emergency.
These may be used in the event of a pressurization malfunction or to “dump” the cabin in the event of smoke or other cabin air contamination.
